not in my experience. i've overclocked every video card i could - and never had any of them degrade, or not clock as high over time. starting w/ tnt2, i put heatsinks on my ram - made a big difference. it doesn't make any difference on a Gladiac however. (i am very sceptical of posts where people claim 'before i put on ramsinks i could only get 370, now i can get it to 4xx' - hmmm, i call bs.)
find the limit of your card's mem (no need to o'c a gts core) and then back it off a bit, eg. my gladiac will run at 388 without artifacts, but gets pretty warm in summer, so i back it off to 380 for safety, when winter rolls around i'll get back on it. also it's a good idea to have good cooling (make a card cooler w/ a 80mm fan=cheap and effective).
with the current generation of cards, boosting mem speeds makes a big difference. you may reduce the effective life of the cards memory from 10 years to 5 years, but i for one have never held onto the same card for much more than a year or so.